tools/lib/: added mechanism to remove USB restrictions (enabling multiple sessions)

- usbopen.h (usb_unrestrict), usbusb.c (usb_unrestrict): drop ID and
  path restrictions
- atusb.c (atusb_open): call usb_unrestrict to remove restrictions

tools/lib/usbopen.c

@@ -228,3 +228,11 @@ void restrict_usb_path(const char *path)
 	else
 		restrict_usb_by_port(path);
 }
+
+
+void usb_unrestrict(void)
+{
+	vendor = 0;
+	product = 0;
+	restricted_path = NULL;
+}